1. Field of the Invention
The present invention relates to a method and related apparatus for controlling a pickup head of an optical disk drive in a feedback manner, and more particularly, to a method and related apparatus for controlling the powers of laser beams emitted by the pickup head in the feedback manner according to a cross-voltage of a laser diode or a laser control signal.
2. Description of the Prior Art
The advantages of light weight, low cost, easy to store, and great data-storing capacity have made an optical disk a most important non-volatile memory media in modern information ages. Data stored in an optical disk is read by an optical disk drive and designing an optical disk drive of good data-reading capability is becoming a popular concern for the information industry.
Please refer to FIG. 1, which is a function block diagram of an optical disk drive 10 according to the prior art. The optical disk drive 10 comprises a motor 14 for rotating a disk 12, a pickup head 16, a control circuit 18, and a processor 20 for controlling functionalities of the optical disk drive 10. The optical disk drive 10 emits laser beams onto the disk 12 with the pickup head 16 and reads data of the disk 12 according laser beams reflected by the disk 12. The control circuit 18 is a pre-amplifier capable of controlling powers of laser beams emitted by the pickup head 16. A laser diode 24, installed in the pickup head 16 as a laser generator, generates the laser beams to project onto the disk 12. A power-adjusting circuit 22 is used to adjust a cross-voltage V between two ends of the laser diode 24 at nodes Np1, Np2. As the cross-voltage V is changed, the powers of laser beams emitted by the laser diode 24 change accordingly. A reference signal end 17A, a cross-voltage output end 17B and a control end 17C are installed in the pickup head 16 for controlling powers of laser beams emitted from the pickup head 16. The power-adjusting circuit 22 receives a control signal 38 transmitted from the control end 17C and adjusts the cross-voltage V according to the voltage level of the control signal 38. A voltage level of the cross-voltage V at node Np1 is determined by a bias voltage 34A output from the reference voltage end 17A. Since the cross-voltage V is controlled by the power-adjusting circuit 22, a voltage level of the cross-voltage V at node Np2 is equivalently determined by the power-adjusting circuit 22. The pickup head 16 outputs a voltage at f3 node Np2 as an output voltage 34B at the cross-voltage output end 17B.
The control signal 38 to control the power-adjusting circuit 22 is generated by the control circuit 18. The control circuit 22 comprises two sub-controllers 26A and 26B, a digital-to-analog converter (DAC) 30, a differential amplifier 32, and three transmission circuits 28A, 28B and 28C. The sub-controller 26A generates a first signal 36A. The processor generates a digital second signal 36B. The digital second signal 36B will be transformed into an analog second signal 36C by the DAC 30 and transmitted to the differential amplifier 32 by the transmission circuit 28C. After respectively receiving the first signal 36A and the analog second signal 36C with a pair of differential-formed input ends (indicated by labels “+” and “−” in FIG. 1), the differential amplifier 32 generates the control signal 38 according to a difference between the first signal 36A and the analog second signal 36C and transmits the control signal 38 to the control end 17C of the pickup head 16 through the transmission circuit 28B. The sub-controller 26B generates the bias voltage 34A. The transmission circuit 28A is used to receive the output voltage 34B output from the cross-voltage output end 17B.
The processor 20 controlling powers of laser beams emitted by the pickup head 16 with the control circuit 18 is described as follows. The control circuit 18 generates the bias voltage 34A, whose voltage level is constant, with the sub-controller 26B to control a voltage level of the pickup head 16 at node Np1 and to keep the voltage level at node Np1 constant. The first signal 36A generated by the sub-controller 26A is also constant. The processor 20 changes the second signal 36B input to the DAC 30 and changes the analog second signal 36C accordingly. Therefore, the control signal 38, which is generated by the differential amplifier 32 according to the difference between the first signal 36A and second signal 36C, is changed according to the variation of the second signal 36C. As the control signal 38 is changed, the power-adjusting circuit 22 adjusts the cross-voltage V accordingly and changes the powers of laser beams emitted by the laser diode 24. In other words, the processor 20 is capable of controlling the power-adjusting circuit 22 to adjust the cross-voltage V and of further controlling the powers of laser beams emitted by the laser diode 24 with the second signal 36B and the control signal 38. Since the bias voltage 34A is constant, the variation of the output voltage 34B reflects the variation of the cross-voltage V.
In the prior art, the processor 20 stabilizes the powers of laser beams emitted from the pickup head 16 by setting the second signal 36B at a constant voltage level. However, the stabilization suffers from a mass production of the control circuit 18. That is, control signals, as well as bias voltages, generated by control circuits in one type are different from each other even if the second signal 36B generated by the processor 20 is kept constant. In addition, the pickup head 16 suffers from the stabilization problem too. Characteristics of the power-adjusting circuits and laser diodes in a pickup head are different from each other even if these devices are of one type. That is, powers of laser beams emitted by different but same type pickup heads are different even if the voltage level of control signal 38 and of the bias voltage 34A are constant. The above-mentioned problem becomes more and more serious as the control circuit 18 and the pickup head 16 combine to function, with the powers of laser beams emitted by the pickup head 16 becoming dramatically unstable. Practically, a laser output power is ideally about 700 μW, but, in reality, the laser output power will raise to as high as 1,400 μW due to a drifting effect caused by the combination of the pickup head and control circuit. As known by those skilled in the art, the optical disk drive 10 not only reads data of the disk 12 according to the laser beams reflected by the disk 12, but controls functions of track-searching, track-locking and pickup head-positioning in a feedback manner according to the reflected laser beams. If the powers of laser beams emitted by the pickup head 16 are extremely high, the powers of laser beams reflected from the disk 12 will raise accordingly. The high power-leveled laser beams will drive transistors of the control circuit 18 to work in borders of a normal-working range, thus distorting signals and malfunctioning the feedback control. On the contrary, if the powers of laser beams emitted by the pickup head 16 are extremely low, too low a power being vulnerable to noise, the optical disk drive 10 cannot function normally either.